Chipotle closed 43 of its Pacific Northwest locations in response to an E. coli outbreak that health officials have connected to the Mexican food chain.

About two dozen people have fallen ill, and more were being tested. At least one lawsuit has been filed by a woman who says she got sick after eating a burrito bowl at a Chipotle in Vancouver, Washington, on Oct. 21.

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Here are some things to know about the outbreak:___WHAT'S BEING DONE TO FIND THE SOURCE?

The investigation began this past weekend. Health officials in Washington state and Oregon have been interviewing the 22 people with a confirmed case of E. coli to find out what they ate. Their blood is being tested to identify the exact strain of the illness.Two people do not believe they ate at Chipotle at all during the past few weeks. They may help provide clues about where the E. coli came from or they could be mistaken, health officials have said.

Food from the affected Chipotle stores also is being tested. Health officials believe the culprit will likely be fresh food such as lettuce or tomatoes, but they won't know for sure until the testing and interviewing is complete.___COULD PEOPLE GET SICK FROM EATING SOMEWHERE ELSE?It's possible all the people who have gotten sick have something else in common, other than Chipotle. It's also possible the full extent of the outbreak has not been discovered.Dr. Scott Lindquist, Washington state epidemiologist, says he's keeping an open mind about the reach of the outbreak but is hopeful officials have stopped the spread of the illness by closing Chipotle restaurants in Washington state and the Portland, Oregon, area.

If the illness is traced to a food supplier, the business may be sending its products to other restaurants or grocery stores. Health officials will not know for sure until they find the source of the E. coli.___HAVE OTHER FOOD-BASED HEALTH PROBLEMS BEEN TIED TO CHIPOTLE?The chain faced a salmonella outbreak linked to tomatoes that sickened dozens of people in Minnesota beginning in August. In California, health workers said norovirus sickened nearly 100 customers and employees at a Chipotle restaurant in Simi Valley in mid-August.Bill Marler, a Seattle food safety lawyer, says three problems within a couple of months means Chipotle isn't paying enough attention to food safety.___WILL CHIPOTLE BE CLOSING MORE RESTAURANTS?So far, the outbreak appears limited to Washington state and Oregon. Chipotle says there is no evidence of a link to other locations, so it won't be closing any more restaurants.The company says it has 1,931 locations, and each restaurant brings in about $2.5 million in revenue a year on average. Chipotle's stock fell 2.5 percent to close at $624 on Monday.___WHAT IS E. COLI AND HOW DO I KNOW IF I HAVE IT?There are hundreds of E. coli and similar bacteria strains in the intestines of humans. Most are harmless, but a few can cause serious problems.Symptoms of E. coli infection include diarrhea, abdominal cramps, nausea and vomiting. Health officials say the best defense against the bacterial illness is to thoroughly wash hands with soap and water.

Marisa D'Angeli, medical epidemiologist with the Washington State Department of Health, encourages anyone who has been sick with intestinal symptoms and has eaten at Chipotle since mid-October to go to the doctor and get tested.

She also said anyone with bloody diarrhea should go to the doctor whether they have eaten at Chipotle or not.